Torlon balls...
Don't take this the wrong way Loren, but...hope you were able to find all your balls once they bounced around the cockpit (as happened to me on a previous boat). Love the Harken Captive Balls for that very reason.
As to the Windward Sheeting Car question, 2 boats later w/ 12+ years w/ this system, all on a mooring ball, I have never experienced the swing/release phenomena you mentioned. I do make it a habit of centering the boom and cleating/winching the port/starboard traveler control lines, which probably helps secure the traveler car. 1st I've heard of this, thankfully.
And now it's off to see the wizard...